The present invention relates to a film thickness testing apparatus and a film thickness testing method in which a substrate, such as being made of a semiconductor wafer, bearing a film on the surface thereof is irradiated with light so as to visually test irregularities in the thickness of the film on the basis of interference caused between light reflected from the surface of the film and light reflected from the surface of the substrate.
Interference of light is utilized in visually testing a thickness of a resist film or the like coated on the surface of a semiconductor wafer. In this testing, the wafer is irradiated with light, so that thickness irregularities of the resist film or the like can be detected on the basis of the intensity of interference. Specifically, as is shown in FIG. 12, interference is caused between light reflected from the surface of a resist film 12 and light having passed through the resist film 12 and reflected from the surface of a wafer 14. When the refractive index of air is indicated by N0, the refractive index of the resist film 12 is indicated by N1, the angle of incidence is indicated by xcex8, the angle of refraction is indicated by xcfx86, and the thickness of the resist film 12 is indicated by t, an optical path difference D between these two reflected lights can be represented as follows:
xcex94=(ABC)xe2x88x92(AD)=2xc2x7N1xc2x7txc2x7cos xcfx86xe2x80x83xe2x80x83(1)
wherein N0xc2x7sin xcex8=N1xc2x7sin xcfx86.
When the optical path difference xcex94 is as large as an integral multiple of the wavelength xcex of the incident light, namely, when
xcex94=mxcex, wherein m=0, 1, 2, 3, etc.xe2x80x83xe2x80x83(2)
the two lights reinforce each other. Alternatively, when the optical path difference xcex94 is as large as a half integral multiple of the wavelength xcex of the incident light, namely, when
xcex94=mxcex, wherein m=1/2, 3/2, 5/2, 7/2, etc.xe2x80x83xe2x80x83(3)
the two lights neutralize each other. Accordingly, when the refractive indexes N0 and N1 and the angle xcex8 of incidence are constant, the intensity of the interference caused between these lights is varied depending upon the thickness t of the resist film 12.
As a light source used for such visual testing, a white light source such as a halogen lamp is used. However, in the spectral distribution of a halogen lamp, comparatively high intensity is distributed in a wide range of wavelength as is shown in FIG. 13. Therefore, when a halogen lamp is used, light respectively having various wavelengths satisfying the interference condition represented by Formula (2) interfere with one another. When such various lights interfere one another, it is difficult to recognize a change in the interference derived from a difference in the thickness t of the resist film. Accordingly, the thickness t of the resist film cannot be accurately detected.
As a method of detecting such a small thickness, a metal halide lamp or the like is used to increase the peak intensity of light, so as to increase the contrast of the interference. In the spectral distribution of a metal halide lamp, however, light intensity in a wavelength range apart from the peak wavelength region is higher than that in a halogen lamp as is shown in FIG. 14. Therefore, the interference is largely affected by the light with a wavelength in a region apart from the peak wavelength region. Also, similarly to use of a halogen lamp, light respectively having various wavelengths satisfying Formula (2) interfere one another. Accordingly, small irregularities in the thickness of the resist film cannot be sufficiently detected even by using a metal halide lamp.
Furthermore, Japanese Laid-Open Patent Publication No. 9-145327 discloses a film thickness measuring apparatus using a low coherent light source. This thickness measuring apparatus comprises interference generating means for generating interference by re-synthesizing divided measurement light and reference light, measurement light means for allowing the measurement light to enter multi-layered plastic sheet, reference light means for allowing the reference light to enter a mirror, and thickness detecting means for detecting the thickness of the multi-layered plastic sheet depending upon the position of the mirror placed for causing the interference, and this apparatus uses a low coherent light source.
Since this thickness measuring apparatus uses the low coherent light source, thicknesses of respective layers in the multi-layered sheet, which is traveling during the measurement, are accurately measured regardless of their thicknesses in a wide range even when the sheet is slightly vibrated or inclined. However, a low coherent light source has a spectral width narrower than that of an incoherent light source such as a halogen lamp as well as an optical output and directivity equivalent to those of a coherent light source such as a semiconductor laser. In other words, such a light source is not sufficiently bright for the visual testing where eyes of humans are used as a light receiving portion. Accordingly, this thickness measurement apparatus cannot be used for the visual testing.
An object of the invention is improving interference caused between light reflected from the surface of a film and light reflected from the surface of a substrate so that irregularities in the film thickness can be accurately and visually tested.
This and other objects of the invention will become more apparent after referring to the following description of the invention considered in conjunction with the accompanying drawings.
The film thickness testing apparatus of this invention comprises a light source part for irradiating a substrate with light having an intensity peak with a half band width (full width half maximum) of 30 nm or less.
Also, the film thickness testing method of this invention comprises irradiating a substrate with light having an intensity peak with a half band width (full width half maximum) of 30 nm or less.